FEATURE ARTICLE 1: OVERWEIGHT AND OBESITY IN ADULTS
Overweight and obesity have become world-wide concerns, reaching epidemic proportions. Obesity is caused by an energy imbalance where energy intake exceeds energy expended over time. This imbalance has been linked to lifestyle factors such as increased consumption of foods with high levels of sugar and saturated fats, as well as a reduction in physical activity.

Overweight and obesity pose a major risk to long-term health by increasing the risk of chronic illnesses such as diabetes, cardiovascular disease and some cancers. It has been estimated that obesity and its associated illnesses cost Australian society and governments a total of $21 billion in 2005.

Data in this article are mainly drawn from the 1995, 2001 and 2004-05 National Health Surveys (NHS), conducted by the Australian Bureau of Statistics (ABS), and refer to adults aged 18 years and over.

BMI rates calculated in this article include people whose BMI was underweight, but excludes those whose BMI was not stated or not known. While BMI is a useful tool for assessing changes in body mass at the population level, it may be less appropriate for certain individuals. For example, it does not account for those with high body mass due to muscle rather than fat.

To account for differences in age structure of the population over time as well as between certain sub-populations, rates and proportions are age standardised to the total estimated resident population at 30 June 2001, where applicable.

In 2004-05, more than half (53%) of all adults, or 7.4 million people aged 18 years and over were either overweight or obese, an increase from 44% (5.4 million adults) in 1995, after adjusting for age differences. The rate of overweight adults has increased from 32% in 1995 to 35% in 2004-05, while the rate of obesity in adults has increased from 12% to 18% over the same period. In each of the NHS conducted since 1995 a higher rate of overweight and obesity was recorded for males compared with females (graph 11.9).

11.9 Adults in normal, overweight and obese BMI categories(a)(b)(c)

Rates of overweight and obesity vary depending on age and sex. Between 1995 and 2004-05, rates of obesity increased for both men and women across all age groups. For men, the largest increase in the obesity rate occurred in the 35-44 year age group which almost doubled from 12% in 1995 to 23% in 2004-05. For women, the increase in the obesity rate was more uniform across age groups (graph 11.10).

Although the NHS collects data at a point in time, it is possible to observe changes over time in the obesity rate for a cohort of people born in the same ten-year period. For example, survey respondents aged 25-34 years in 1995 and those aged 35-44 years in 2004-05, while not the same respondents, are seen as representing the same group of people who have aged ten years. Among the male cohorts, the greatest increase in the rate of obesity occurred for the group aged 35-44 years in 1995 (12% in that year compared with 25% ten years on, in 2004-05).

For females, the greatest increase in the obesity rate occurred for the cohort aged 35-44 years in 1995, with 12% classified as obese compared with 20% of the 45-54 year olds representing the same group of people in 2004-05.

The NHS can provide insight into associations between certain socio-demographic characteristics and excess weight (table 11.11). As some of the populations discussed have differing age structures, the proportions presented in this section are age standardised to remove the confounding influence of age.

Most people born overseas are in good health on arrival in Australia due to the rigorous health checks they undergo to be eligible for migration. This 'healthy migrant effect' generally wanes as their length of time in Australia increases, and time since migration is an important factor in excess weight in migrants.

In 2004-05, the overall adult obesity rate was 18%. People born overseas who arrived before 1996 had a slightly lower age-standardised rate of obesity (15%) compared with people born in Australia, while the rate was even lower (10%) for more recent arrivals (between 1996 and 2005).

Adults with a degree, diploma or higher qualifications were less likely to be obese than those with other or no post-school qualifications. In 2004-05, around one-fifth (20%) of those without a non-school qualification, and 19% of those with other non-school qualifications (i.e. trade certificate), were classified as obese. By comparison, 13% of those with a degree/diploma or higher qualification were classified as obese.

While equal proportions (53%) of people in low income and high income households were overweight or obese in 2004-05, those in low income households were more likely to be obese. Around a fifth (21%) of adults in low income households were obese compared with 15% of adults in high income households.

The Socio-economic Indexes for Areas Index of Disadvantage provide a method for comparing areas based on the characteristics of the people that live in them (such as income, employment and education). In 2004-05, adults living in areas of greatest relative disadvantage had a higher age-standardised rate of obesity (22%) compared with adults living in areas with the lowest relative disadvantage (13%).

Aside from socio-economic differences between areas in terms of income, employment and education, some areas may also offer greater opportunities for physical activity and greater access to healthy food options.

(a) Sub-populations age standardised to estimated resident population at 30 June 2001.

(b) Based on self-reported height and weight.

(c) Includes persons whose BMI was underweight and excludes persons whose BMI was not stated or not known.

(d) Gross weekly equivalised household income. Low income households are in the lowest quintile, middle income in the third quintile and high income in the highest quintile of household income.

(e) The first quintile contains areas with the greatest relative disadvantage and the fifth quintile contains those areas with the lowest relative disadvantage.

Source: ABS data available on request, National Health Survey.

In 2004-05 the rate of obesity in Outer Regional/remote areas was 22%, while in Major Cities and Inner Regional areas the rates were 17% and 19% respectively. The rate of overweight was similar across the remoteness areas (36% in Outer Regional/remote areas of Australia, compared with 35% in Major Cities).

End note

1. Body Mass Index (BMI) scores are calculated from reported height and weight information, using the formula: weight in kilograms divided by the height in metres squared. BMI values are grouped as follows: underweight (less than 18.5); normal weight (18.5 to less than 25.0); overweight (25.0 to less than 30.0) and obese (30.0 or higher).

Self-reported height and weight may also differ from measured height and weight. In 1995, a comparison of these two methods suggested that when self-reporting, people tend to overstate their height and understate their weight. For further details, see How Australians Measure Up, 1995(4359.0).